Recently, a method for improving output of a motor for improving performance of a hybrid vehicle has been actively researched and developed. It is generally known that the output of the motor is proportional to the number of turns of a coil wound around a stator core. Accordingly, in order to improve the output of the motor without increasing a size of the motor, a method of increasing a space factor of the coil wound around the stator core may be considered. As one of the aforementioned methods, a method of winding a polygonal coil having a shape of an approximate quadrangular cross section, instead of using an annular coil having a circle cross section, has been recently researched.
However, the polygonal coil has relative difficulty in an operation of coil winding compared to the annular coil. Accordingly, as a method of easily winding the polygonal coil, a motor (hereinafter, referred to as a “hairpin winding motor” for convenience of description) using a method of forming a coil winding portion by inserting a plurality of separated hairpins in a coil of a stator to be fastened to the coil of the stator, and then connecting the respective hairpins by welding has been suggested. The aforementioned hairpin winding motor forms coil winding of the stator core by fastening the hairpins shaped like an approximate U-shape to respective slots of the stator core and then connecting the hairpins arranged in respective layers of the respective slots by welding. Accordingly, the hairpin winding motor overcomes an apparatus limit according to a winding apparatus, and may easily perform a coil winding operation with the polygonal coil.
However, the hairpin winding motor accompanies a plurality of repeated welding operations differently from a winding method by a winding apparatus, so that the repeated welding operations primarily cause defective connection between the respective hairpins or degrade productivity.